Polyacetal resin is used in diverse fields as an engineering plastic due to its excellent physical properties, such as mechanical, electrical and chemical properties (e.g., chemical and heat resistances). However, the identification of new end-use applications for polyacetal resins often requires further property alterations and/or enhancements.
One such property that is needed when polyacetal resin is employed to form molded parts used for automotive trim or optical instruments is a relatively low surface gloss. A low surface gloss will thus reduce the light reflectivity property of the molded part. As a result, low surface gloss on molded parts will tend to cause less eye irritation (i.e., since less light will be reflected from the part's surface). Furthermore, low surface gloss on molded parts fabricated from engineering plastics tends to impart a more aesthetically pleasing high-grade appearance to the part.
An ability to "engineer" the surface gloss characteristics would also be beneficial in those instances where molded parts fabricated from polyacetal resin (which inherently exhibits a relatively high surface gloss) are used in cooperation with molded parts formed of other non-polyacetal resins. That is, it would be especially desirable to reduce the surface gloss of polyacetal resin to an extent where it is substantially equivalent to the molded parts of non-polyacetal resin in which it is cooperatively used. An inability to date to reduce the surface gloss of polyacetal resins by controlled amounts so that it is visually imperceptible with parts molded from non-polyacetal resins has thus been one barrier to using polyacetal resin in end-use applications (e.g., as parts for electric and electronic devices) where the polyacetal part will be visible.
Attempts in the past to reduce the inherently high surface gloss of polyacetal resins have included incorporating inorganic filler material, such as calcium carbonate or talc, in a polyacetal base resin. However, large amounts of such inorganic filler material must typically be added to the polyacetal base resin before the desired low surface gloss appearance is realized. The addition of relatively large amounts of inorganic filler material, however, is disadvantageous since it has a tendency to reduce the mechanical characteristics (especially elongation and toughness) normally associated with polyacetal resins. Thus, for example, the mechanical properties may be degraded by the addition of relatively large amounts of filler material to the point where molded parts of such a highly filler-loaded polyacetal resin breaks during assembly or when dropped.
It has also been proposed to lower the surface gloss characteristics of polyacetal articles by incorporating an organic filler material in the form of cross-linked acrylic resin particles. However, this technique is less than satisfactory in terms of the resulting gloss reduction that is achieved. Furthermore, the addition of acrylic resin increases the likelihood that the polyacetal resin may decompose when the acrylic resin particles are blended, kneaded or melt-blended.
Thus, what has been needed is a polyacetal resin composition having reduced surface gloss characteristics which can be "engineered" without affecting the desirable physical properties of polyacetal resin generally. It is towards fulfilling such a need that the present invention is directed.
Broadly the present invention is directed to polyacetal resin compositions which include an effective surface gloss reducing amount of a novel core-shell polymer as a surface gloss reduction (SGR) agent, and to the core-shell polymer itself. In this connection, the core-shell polymer which is employed as the SGR agent is one which has a rubbery polymer core and a glassy (amorphous) polymer shell comprised of a vinyl copolymer having an oxygenic polar group. In preferred embodiments, the low-gloss polyacetal resin compositions of this invention include between 1 to 50 parts by weight of the core-shell polymer per 100 parts by weight of the polyacetal base resin.
Further aspects and advantages of this invention will become more clear from the following detailed description of the preferred exemplary embodiments thereof.